The invention relates to producing shale oil and related mineral materials from subterranean deposits of oil shale.
Numerous subterranean oil shales are mixed with water-soluble minerals. Such shales comprise substantially impermeable, kerogen-containing, earth formations from which shale oil can be produced by a hot fluid-induced pyrolysis or thermal conversion of the organic solids to fluids. A series of patents typified by the T. N. Beard, A. M. Papadopoulos and R. C. Ueber U.S. Pat. Nos. 3,739,851; 3,741,306; 3,753,594; 3,759,328; 3,759,574 describe procedures for utilizing the water-soluble minerals in such shales to form rubble-containing caverns in which the oil shale is exposed to a circulating hot aqueous fluid that converts the kerogen to shale oil while dissolving enough mineral to expand the cavern and expose additional oil shale.
In such oil shales significant amounts of time and energy may be consumed in forming a rubble-containing cavity. Such deposits are both impermeable and poor conductors of heat. Even where such an oil shale is relatively rich in water-soluble mineral, years of circulating hot aqueous fluid may be required to leach out a rubble-containing cavity that exposes enough oil shale to be economically useful. In such operations the rates of rubbling and leaching become slower as the cavity becomes larger. And, such rates are slower where the original concentration of soluble minerals in the oil shale is lower.
In general, such a rubble-containing cavity should have a radius in the order of at least about 50 feet; preferably, at least about 100 feet. The cavity height should approximate the thickness of the oil shale deposit and should be at least about 200 feet; preferably, at least about 500 feet. The leaching and dissolving action should also be made severe enough or continued long enough to cause chunks of oil shale (in the cavity and along the inner walls of the cavity) to have a permeability of at least about 1 and preferably at least about 10 darcies.
In a typical deposit, the oil shale formation might be about 500 feet thick, might contain about 25 percent by weight nahcolite in a relatively uniform distribution, and might contain enough kerogen to provide a Fischer assay richness of about 25 gallons per ton. A borehole might be drilled through the oil shale and under-reamed or drilled to a diameter of about 2 feet. If a relatively fresh water, heated to about 300.degree.F, were to be circulated through the cavity at about 10,000 barrels per day, about 4 years would probably be required to form a rubble-containing cavity having a diameter of about 200 feet and a height of about 500 feet. Such a rate of growth might be improved somewhat by a cavity-expanding and solids-removing optimization of the type described in the T. N. Beard and P. VanMeurs U.S. Pat. No. 3,779,602 and/or the L. H. Towell and J. R. Brew U.S. Pat. No. 3,792,902. In those processes the hot water is circulated at a pressure optimized for enhancing growth without unduly increasing carbonate precipitation and/or fresh aqueous fluid is mixed with downhole portions of outflowing solution to prevent precipitation in the production tubing string as the pressure and temperature decrease at shallower depths.
Text books published as early as 1946 indicate that "well shooting", to create permeability by detonating explosives was used where the reservoir rocks were hard and well cemented; see "Petroleum Production Engineering, Oil Field Development, also Oil Field Exploitation" by L. C. Uren, McGraw Hill Book Company, 1946. U.S. Pat. No. 3,533,471 indicates that, in hard rock, the extent of fragmentation can be improved by forming generally parallel fractures, propping them open with a granular propping material, injecting as explosive liquid into one, and detonating the explosive so that the shock waves are reflected from the discontinuity formed by the other fracture. The R. T. McLamore U.S. Pat. No. 3,637,020 describes the use of conical notches and shaped explosive charges to enhance the tendency for fractures, in hard formations, to be horizontal. U.S. Pat. Nos. 3,561,532; 3,587,744 and 3,593,793 describe various explosive liquids and methods of emplacing them in hard formations to increase permeability by well shooting. The P. J. Closmann and Helmer Ode U.S. Pat. No. 3,448,801 describes an enlargement of a nuclear chimney within a subterranean oil shale. Lower energy explosives in surrounding locations are detonated between the time the chimney is expanding radially outward and the time its roof collapses into rubble.
Subterranean oil shales, such as those containing or associated with water-soluble minerals, are relatively highly compressible. Compressible materials tend to absorb or damp-out direct or reflected shock waves without undergoing much if any fragmentation. However, the present invention is, at least in part, premised on the discovery that in such a subterranean oil shale formation, the combination of (a) the relatively high degree of shock wave energy reflection from a cavity that is substantially free of any solids that are rigidly held by lithostatic pressure, and (b) the relatively free movement of displaced and reoriented rock pieces into such a cavity, can cause enough fragmentation to significantly enhance the rate at which a rubble-containing cavity can be solution-mined before or while shale oil is recovered.